1. Field of the Invention
The present invention intends to reduce the expense necessary in the treatment of used solution and to recover useful materials, such as, tin by circulatingly employing the used solution of organic electrolyte.
2. Description of the Prior Art
In the manufacture of a tin-plated steel strip, the tin-plated material is usually immersed in an organic electrolyte solution or, alternatively, the same solution is sprayed to the material for surface conditioning before the tin-melting step. In either the immersion or the spraying treatment of electrolytic tin plated materials, tin accumulates in the organic electrolyte solution by being dissolved from the material or by transferring from the preceding treating solution. The accumulated tin may be likely to reduce the efficacy of the organic electrolyte solution and, as a result, a lustrous surface will not result in the final melting treatment.
The same applys when the organic electrolyte solution is sprayed onto an electrolytic tin plated material and the excess of the solution is collected for repeated use. When the efficacy of an organic electrolyte solution is lost because of accumulated tin, this must be replaced by a fresh solution of the same organic electrolyte. However, since the exhausted solution is still strongly acid and has high COD values, it can not be discarded as it is. A necessary remedy is, for example, to neutralize the solution with lime to precipitate the tin in the form of the hydroxide and chemically oxidizing the filtrate. Also, an oxidative decomposition treatment may be applied to the solution by a biochemical means before being discarded in a non-polluting form. However, such treatments necessarily require large capital expenditures for the plant and high consumption of chemicals, and also an increased floor space for the plant. Even so, useful materials, such as, tin and the organic electrolyte are lost.
On the other hand, attempts have been made to regenerate an organic electrolyte solutions having a high concentration of tin. One of the methods involves concentration of the solution in a vacuum evaporator followed by transfer to the plating bath. This method requires a large amount of heat energy for evaporation. In addition, some of the solutions must be discarded owing to the unbalanced ratio in the amounts of tin and the organic electrolyte.
Another category of the methods is concerned with recovering tin by electrolysis. Since this method is intended to recover concentrations of tin lower than 0.1 g/l by electrolysis, emphasis is laid on how to improve the current efficiency in the electrolysis of a low concentration solution, and methods and apparatuses have been developed. However, these still involve high treatment costs.
2. Summary of the Invention
The present invention has been developed to overcome the above mentioned difficulties, and relates to a circulative treatment process of a used tin plating solution, in which tin in the used solution of an organic electrolyte is removed prior to the melting treatment in the electrolytic tin plating process and the resulting solution is further circulated for treating the tin-plated materials comprising extracting tin in the used solution by electrolysis to maintain the concentration of tin in the range from 0.2 to 5.0 g/l.
According to the present invention, the used organic electrolyte solution from which tin has been recovered by electrolysis is circulatingly supplied to effectively treat tin-plated steel strip and the tin recovered can be used for the proper purpose without substantial loss of the natural resource. The apparatus necessarily required for the circulative treatment of this invention basically include those of compulsory circulation of the fluid and electrolysis, which cost much less than the prior art apparatuses for the regeneration and treatment to overcome the pollution problem.